How can six and half feet of DNA be folded into the tiny nucleus of a cell? Researchers funded by the National Institutes of Health have developed a new imaging method that visualizes a very different DNA structure, featuring small folds of DNA in close proximity. The study reveals that the DNA-protein structure, known as chromatin, is a much more diverse and flexible chain than previously thought. This provides exciting new insights into how chromatin directs a nimbler interaction between different genes to regulate gene expression, and provides a mechanism for chemical modifications of DNA to be maintained as cells divide. The results will be featured in the July 28 issue of Science. Computer enhanced electron micrograph image of interacting chromatin chains of 5 to 24 nanometers in the cell nucleus. Credit: Clodagh C. O’Shea, Salk Institute, La Jolla, CA

For decades, experiments suggested a hierarchical folding model in which DNA segments spooled around 11 nanometer-sized protein particles, assembled into rigid fibers that folded into larger and larger loops to form chromosomes. However, that model was based on structures of chromatin in vitro after harsh chemical extraction of cellular components.

Now, researchers at the Salk Institute, La Jolla, California, funded by the NIH Common Fund, have developed an electron microscopy technique called ChromEMT that enables the 3D structure and packing of DNA to be visualized inside the cell nucleus of intact cells. Contrary to the longstanding text book models, DNA forms flexible chromatin chains that have fluctuating diameters between five and 24 nanometers that collapse and pack together in a wide range of configurations and concentrations.

The newly observed and diverse array of structures provides for a more flexible human genome that can bend at varying lengths and rapidly collapse into chromosomes at cell division. It explains how variations in DNA sequences and interactions could result in different structures that exquisitely fine tune the activity and expression of genes.

“This is groundbreaking work that will change the genetics and biochemistry textbooks,” remarks Roderic I. Pettigrew, Ph.D., M.D., director of the National Institute of Biomedical Imaging and Bioengineering (NIBIB), which administered the grant. “It’s an outstanding example of how constantly improving imaging techniques continue to show the true structure of everything from neuronal connections in the brain to the correct visualization of gene expression in the cell. It reveals how these complex biological structures are able to perform the myriad intricate and elaborate functions of the human body.”

“We identified a fluorescent small molecule that binds specifically to DNA and can be visualized using advanced new 3D imaging methods with the electron microscope,” explained Clodagh O’Shea, Ph.D., the leader of the Salk group, associate professor and Howard Hughes Medical Institute Faculty Scholar. “The system enables individual DNA particles, chains and chromosomes to be visualized in 3D in a live, single cell. Thus, we are able to see the fine structure and interactions of DNA and chromatin in the nucleus of intact, live cells.”

Dr. O’Shea’s team included collaborators from the University of California, San Diego, and the National Center for Microscopy and Imaging Research, San Diego.

The researchers believe their discovery dovetails with their research on how tumor viruses and cancer mutations change a cell’s DNA structure and organization to cause uncontrolled cell growth. It could enable the design of new drugs that manipulate the structure and organization of DNA to make a tumor cell ‘remember’ how to be normal again or impart new functions that improve the human condition.

“To see the human genome in in all of its 3D glory is the dream of every biologist. Now, we are working to design probes that will allow us to also see the proteins that bind to the DNA to turn genes on and off. We will then be able to view an actual gene in action,” concluded O’Shea.

The research was supported by NIH grants from the National Institutes of Health Common Fund (U01EB021247), the National Cancer Institute and the National Institute of General Medical Sciences. Additional funding was provided by the Howard Hughes medical Institute and the W.M. Keck Foundation.

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About the National Institute of Biomedical Imaging and Bioengineering (NIBIB): NIBIB’s mission is to improve health by leading the development and accelerating the application of biomedical technologies. The Institute is committed to integrating the physical and engineering sciences with the life sciences to advance basic research and medical care. NIBIB supports emerging technology research and development within its internal laboratories and through grants, collaborations, and training. More information is available at the NIBIB website: http://www.nibib.nih.gov.

About the National Institute of General Medical Sciences: (NIGMS) supports basic research that increases understanding of biological processes and lays the foundation for advances in disease diagnosis, treatment and prevention. NIGMS-funded scientists investigate how living systems work at a range of levels, from molecules and cells to tissues, whole organisms and populations. The Institute also supports research in certain clinical areas, primarily those that affect multiple organ systems. To assure the vitality and continued productivity of the research enterprise, NIGMS provides leadership in training the next generation of scientists, in enhancing the diversity of the scientific workforce, and in developing research capacities throughout the country. https://www.nigms.nih.gov

About the National Cancer Institute (NCI): NCI leads the National Cancer Program and the NIH's efforts to dramatically reduce the prevalence of cancer and improve the lives of cancer patients and their families, through research into prevention and cancer biology, the development of new interventions, and the training and mentoring of new researchers. For more information about cancer, please visit the NCI website at cancer.gov or call NCI's Cancer Information Service at 1-800-4-CANCER.

About the Common Fund: The NIH Common Fund encourages collaboration and supports a series of exceptionally high-impact, trans-NIH programs. Common Fund programs are designed to pursue major opportunities and gaps in biomedical research that no single NIH Institute could tackle alone, but that the agency as a whole can address to make the biggest impact possible on the progress of medical research. Additional information about the NIH Common Fund can be found at http://commonfund.nih.gov

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 institutes and centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit www.nih.gov.

The mission of the National Institute of Biomedical Imaging and Bioengineering (NIBIB) is to improve health by leading the development and accelerating the application of biomedical technologies. The Institute is committed to integrating the physical and engineering sciences with the life sciences to advance basic research and medical care.